Japanese unexamined patent application publication 2-291655 discloses a photomultiplier tube having a circular cage type electron multiplying unit. In the circular cage type electron multiplying unit, a path formed in the spaces between opposed dynodes traces a circle around an axis orthogonal to the tube axis. The second dynode and the anode are positioned on opposing ends in relation to the tube axis. Accordingly, the photomultiplier tube can be contracted in its axial direction, reducing the overall size of the tube construction.
In order to form a circular path in the spaces between opposed dynodes, concave dynodes are positioned on the outer side of the path, while dynodes having a substantially flat surface are arranged on the inner side of the path, wherein the inner dynodes have a smaller surface area than those of the outer dynodes. The anode is pole-shaped, but configured to encompass the last dynode. This anode has exceptional resistance to vibration due to its pole shape.
However, since the surface area of dynodes positioned on the inner side of the path is smaller than that of the dynodes disposed on the outer side of the path, the electron density increases near the dynodes positioned on the inside of, the path. Since electrons are sequentially multiplied as they approach the anode, the electron density near the penultimate dynode becomes extremely high, as the surface area of this dynode is small. Accordingly, an undesirable space charge effect can easily occur. Moreover, with its pole shape, the anode has weak electric field intensity. The circular cage type electron multiplying unit construction disclosed in Japanese unexamined patent application publication 2-291655 has poor pulse linearity due to the problems of the space charge effect and the weak electric field intensity. That is, the output signal does not increase linearly with the increase in inputted optical intensity. Rather the output signal level drops.
A photomultiplier tube designed to improve pulse linearity is disclosed in Japanese unexamined patent application 7-245078. As in the photomultiplier tube of Japanese unexamined patent application publication 2-291655 described above, this publication provides a photomultiplier tube having a shortened length along the tube axis by providing the second dynode and the anode on opposing ends of the tube. In this photomultiplier tube, the electron multiplying unit having a plurality of dynode includes a first section formed of a plurality of box and grid dynodes and a second section formed of a plurality of line focus dynodes. The path formed in the spaces between facing dynodes is curved by arranging box shaped dynodes in the first section, while the path formed in the spaces between facing dynodes in the second section follows a straight line using the plurality of line focus dynodes. The anode has a flat mesh shape and is positioned between the last dynode and the penultimate dynode.
With this construction, line focus dynodes having an equivalent size are used in the second section where the electron density is increasing, thereby preventing an increase in electron density and restraining the space charge effect.
Since the anode in the photomultiplier tube of Japanese unexamined patent application publication 7-245078 has a mesh shape, it is possible to increase the intensity of the electric field. Hence, this photomultiplier tube can improve pulse linearity, unlike the circular cage type electron multiplying construction of Japanese unexamined patent application publication 2-291655.
As described above, the photomultiplier tube of Japanese unexamined patent application publication 7-245078 uses box-shaped dynodes in the electron multiplying unit. These box-shaped dynodes have a large and complex construction including a box-type secondary electron emitting unit and a grid, making it difficult to manufacture a photomultiplier tube that is compact in size with a capacity to withstand vibrations.
Further, the travel time of the electrons increases among these box-type dynodes, resulting in an insufficient time response.
In view of the foregoing, it is an object of the present invention to provide a photomultiplier tube having good vibration resistance and pulse linearity characteristics and being capable of improving time response.